How to report an account being boosted?

I played an a game where the enemy team won because of the jungler, he had an incredible KDA and played everything perefectly so I searched him on to find out he is boosting the account for somebody, the reason I believe this is because the account has a 49% WR in ranked but when you go to the "ranked" tab on his account it only shows the last 11 games he played all in the last 3 days and his average kda in the last 11 games that I can see is 18/8/11, he has managed to lose 3 games but as you can see his KDA in all 3 games they were really good and had an average 70% Kill participation and he only lost those games because his team fed super hard. There was one game where he won but fed 3/10/12, I believe this is the original owner of the account as in this game he played a completely different lane and champ. Have a look for yourself, go to the ranked tab:
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